The present invention relates to CMOS image sensors used in scanners, and more particularly, to a readout circuit in the CMOS image sensor that draws no DC current during readout.
Scanners are commonly used in connection with a personal computer (PC) to digitize a document. The document may be a textual document or other type of document, such as a photograph. One of the important components of a scanner is the imaging device. In many modem scanners, the imaging device is a CCD image sensor. Recently, CMOS image sensors have made significant inroads into applications previously dominated by CCD image sensors. This is due in part to the lower cost and lower power consumption of CMOS image sensors. These advantages are particularly important in PC camera applications, security applications, cell phone applications, and the like.
Depending upon the particular application, CMOS image sensors come in a variety of array sizes. High-resolution image sensors with over one million pixels are used in digital still cameras, while lower resolution CIF, VGA, or SVGA formats are used for security camera or PC camera applications. In many applications, the pixel array size is on the order of 352-1280 pixels per row with 288-1024 pixels per column.
For scanner applications, the pixel array has significantly different dimensions. Typically, approximately 10,000 pixels are in each row. Specifically, most scanners are manufactured to scan documents 8.5 inches wide. At a resolution of 1200 dots per inch (dpi), this requires a little over 10,000 pixels. Further, a black and white scanner will only require a single row of pixels. However, for a color scanner, three rows of 10,000 pixels are required, one row for the color red, one row for the color green, and one row for the color blue.
During readout of the signals from each pixel in the array, there is typically a current associated with the readout process. If a large number of pixels must be read out simultaneously, then a large current is required. The large current required will also cause a voltage drop in the power supply line and also affect the ground line of the image sensor. This in turn will cause non-uniformity and decrease in the signal range of the image sensor.
While this is an issue for many image sensors, the problem is complicated by the need to read out over 30,000 pixels simultaneously for a color image sensor used in a scanner. Moreover, because of the large number of pixels in a row, the actual dimension of the pixel array is on the order of 2 centimeters. Because of this, the metal lines used as power and ground are unusually long compared to image sensors used in other applications. For this additional reason, the voltage drop in the power and ground lines is problematical.